1. Field of the Invention
The present invention relates to an eddy current flaw detection apparatus for the detection of flaws or contaminating foreign material in metal members.
2. Prior Art
In the production processes of copper wiring or the like, if there is contaminating foreign matter, such as a piece of iron or the like, within the copper wiring material, or if there is a flaw in this material, this can result in disconnection during the wire drawing process. For this reason, it is necessary to determine whether foreign matter or flaws or the like are present before the wire drawing process.
Conventionally, in this type of detection, an eddy current flaw detection apparatus such as that shown in FIGS. 6 to 8 was used. Reference numeral 1 indicates a traveling conductor; detection coils La and Lb, which are coaxial with conductor 1, and do not make contact with conductor 1, are disposed in a separated manner. Detecting coils La and Lb are connected so as to form the two sides of an alternating current bridge circuit, and in the case in which there is no flaw in the conductor 1 which passes through detecting coils La and Lb, the output signal between terminals a and b is so adjusted as to have a zero balance.
In accordance with this apparatus, in the case in which a flaw or foreign matter passes through detecting coils La and Lb, the eddy current within conductor 1 changes, a difference occurs in the inductances of detecting coils La and Lb, and the zero balance is disrupted, so that an output signal is generated between terminals A and B, and it is possible to conduct the detection of flaws or foreign matter based on this signal.
A structure in which the detecting coils La and Lb are connected in a differential manner so that the directions of their magnetic fields are opposed, as shown by the arrows in FIG. 8, is preferable to a structure in which the detecting coils La and Lb are connected in an accumulated manner so that the directions of their magnetic fields are in phase, as shown by the arrows in FIG. 7.
When this type of differential connection is used, if for example, the conductor 1 becomes eccentric within the detecting coils La and Lb, or the composition of conductor 1 changes, and noise signals are generated in detecting coils La and Lb, these noise signals cancel each other out, the noise component of the output signal is reduced and the SN ratio can be increased.
However, in the above described eddy current flaw detection apparatus, the detecting coils La and Lb normally produce opposing magnetic fields, so that these magnetic fields repel each other, and it is difficult for the magnetic fields to penetrate sufficiently into the interior of conductor 1, so that the partial detection sensitivity is reduced. As a result, in cases in which small flaws or small amounts of foreign matter are position near the axial center of the coil, it is difficult to detect them.